1. Field of the Invention
This invention relates to image forming apparatus and, more particularly, to an image forming apparatus for forming a color image by superposing a plurality of colors.
2. Description of the Prior Art
An electrophotography typelaser beam printer is known as an apparatus of this kind. This type of printer is capable of forming a half-tone image while achieving high resolution and good gradation characteristics by using a density signal obtained by pulse-width code modulation (PWM), although it is based on a binary recording system.
However, if the PWM method is adopted, a problem occurs relating to image formation at very small density in a highlighted portion of an image. That is, referring to FIG. 8 which is a diagram of the timing of dot formation in accordance with the PWM method, if the maximum time width of a PWM signal f is T, the width h of a spot of laser light is to be changed according to the PWM signal width t as the signal width t changes between O (white dot) and T (black dot). However, the substantial laser driving force is, in fact, as represented by a waveform g depending upon operating characteristics of the semiconductor laser element and the circuit for driving the same. In addition, the semiconductor laser element does not emit light unless the supplied current exceeds a predetermined threshold level Ish (FIG. 9). There is no problem when the PWM signal width is T (full tone) or t.sub.2 (half-tone), but, when the PWM signal width is t.sub.3 (highlight tone), it is uncertain whether the laser emission is actually on or off with respect to the corresponding spot width h. Even if it is on, density formation based on the electrophotography method is very unstable in this case, and it is not possible to effect density formation with stability. When the PWM signal width is t.sub.4, image density formation is not effected at all. Consequently, no black dots are formed with respect to highlighted portions of a PWM signal width of t.sub.3 (e.g., 10 ns) or less.
To solve this problem, the applicant of the present invention has already proposed (in U.S. Ser. No. 07/423,383, filed Oct. 19, 1989) an image formation method in which the density of a highlighted portion is concentrated on a certain pixel in a block (density concentration method).
FIGS. 13A to 13D are diagrams showing an example of application of the density concentration method to multiple color image formation. FIG. 13A illustrates a case in which the direction of density concentration is fixed (e.g., to the left). As illustrated, in a highlighted portion of an image, both the density of color .largecircle. and the density of color x are concentrated on the left hand side. The density of the highlighted portion, which may be lost by the conventional method, can be substantially reproduced if these two colors are superposed. However, if the superposing printing of the color X is shifted rightward to an extent corresponding to one dot as shown in FIG. 13B, the two colors are not superposed at all and the difference between the color tones based on the conditions shown in FIGS. 13A and 13B is very large. Since the dot formation density in the highlighted portion is small, the change in the color tone is particularly sensible.
FIG. 13C illustrates a case in which the direction in which the direction of density concentration is random. As illustrated, the density of color .largecircle. and the density of color x are concentrated in random directions. Accordingly, the lost density of the highlighted portion is reproduced at a certain rate by superposing these two colors. However, if the superposing printing of the color x is shifted rightward to an extent corresponding to one dot as shown in FIG. 13D, the rate at which the two colors are superposed varies and the color tone is therefore unstable as between the conditions shown in FIGS. 13C and 13D.
FIGS. 14A to 14E are diagrams showing states of dot formation of highlighted and half-tone portions of an image. Highlight data items (representing a pixel density of, e.g., 15) shown in FIG. 14A are concentrated on, for example, the upper-left pixels (with a pixel density of 60) in respective unit blocks (each containing 2.times.2 pixels), as shown in FIG. 14B, and are recorded as dots with a spot diameter in accordance with the pixel density of 60, as shown in FIG. 14C. However, since the dot formation density (number of dots per unit area) in each block of the highlighted portion is reduced to 1/4 in this manner, the change in the color tone caused by an error in color superposition at this portion is sensible.
On the other hand, half-tone data items (representing a pixel density of, e.g., 60) shown in FIG. 14D are directly used for dot recording, as shown in FIG. 14E. Since the dot formation density of this half-tone portion is large, the change in the color tone is inconspicuous. Thus, it is important to cope with the problem of changes in color tones of highlighted image portions.